Hospital crowding is a fundamental challenge confronting U.S. healthcare systems. Although it has been recognized as a problem in multiple aspects of patient care, most studies on crowding have focused on the emergency department (ED), showing increased mortality, reduced quality of care, and impaired access to care during periods of crowding. From the inpatient perspective, many studies showed the adverse effects of high patient-to-nurse ratios on outcomes. These and other studies were the basis for California (CA) legislation in 2004 requiring fixed patient-to-nurse ratios at all non-federal, acue care hospitals. Few studies have investigated the impact of this staffing benchmark on patient outcomes, showing mixed results. Also, only a few studies have examined the effect of a surge in admissions, high inpatient occupancy, and increased demand for resources on outcomes, demonstrating mixed results. Most of these studies have looked at the impact of high patient-to-nurse ratios and high hospital occupancy on outcomes for adults in general, which may mask the impact on medically vulnerable populations, including the elderly, who may be most sensitive to any adverse effects. The long-term goal of this work is to inform hospital administrators and policymakers about the potential adverse effects of hospitals operating at high occupancy levels to guide areas where interventions may be implemented to promote safer clinical practice, improved quality of care, and better patient outcomes for medically vulnerable populations. The overall objective, essential in attaining our long-term goal, is to fill the knowledge gap regarding the impact of high hospital occupancy on clinical outcomes for elderly Medicare patients. Our central hypothesis is that high occupancy produces adverse outcomes for elderly Medicare inpatients. We will use administrative hospital discharge data for the state of CA and Arizona (AZ) for 1998- 2011, including the 6 years before and 7 years after the implementation of state law in CA (2004) requiring fixed patient-to-nurse ratios. We will objectively test our central hypothesis through the following Specific Aim. Determine the impact of high hospital occupancy on outcomes for elderly Medicare inpatients. The underlying rationale for this proposal is that by identifying the subset of patients and outcomes that may be adversely affected by high occupancy, this study can have a positive impact by informing hospitals about targeted interventions aimed at improving outcomes for some of their most medically vulnerable inpatients.